Seismology is used for exploration, archaeological studies, and engineering projects that require geological information. Exploration seismology provides data that, when used in conjunction with other available geophysical, borehole, and geological data, provides information about the structure and distribution of rock types and their contents. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons and ores. Most oil companies rely on exploration seismology to select sites in which to drill exploratory oil wells.
Traditional seismology employs artificially-generated seismic waves to map subsurface structures. The seismic waves propagate from a seismic energy source down into the earth and reflect from boundaries between subsurface structures. Surface receivers detect and record reflected seismic waves for later analysis.
The seismic waves are usually generated by energizing the earth with a suitable source of seismic wave energy. Most commonly, the seismic energy sources create elastic waves involving earth particle movement in the direction of wave propagation. These waves, referred to as compressional or longitudinal waves, are readily generated by delivering a vertical impact against the earth's surface with an explosion or a mechanical transducer. The technical literature has expressed the utility for seismic prospecting employing a second form of wave, termed shear waves, wherein the earth particle motion is orthogonal to the direction of wave propagation. Shear waves have slower rates of propagation through the earth than do longitudinal waves so they can produce a higher degree of resolution at a given frequency. This may enable the detection of subterranean anomalies that might otherwise be undetectable and the mapping of larger bodies with a higher degree of precision. Horizontally polarized shear waves are also less likely to be converted into different wave types upon interacting with horizontal interfaces as is the case with compression waves and accordingly seismograms made from such waves may be simpler to interpret.
Despite these recognized advantages, the use of shear waves in seismic prospecting has been greatly limited because of the unavailability of suitable shear wave energy sources. The difficulty encountered in designing such energy sources involves the manner of coupling an impact to the earth so that it will impart the desired shear motion to the earth's surface. Thus far, shear wave energy sources have proven to be infeasible or lacking in sufficient reproducibility, frequency band width, and power for repeated high resolution surveys for, e.g., reservoir monitoring.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed by the scope of the appended claims